The invention relates to a tuning method used in a base station which is composed of at least one transceiver unit and a filtering means which is separate from the transceiver unit and filters signals which are generated on the transmitting end of the transceiver unit, of which signals part advances to the filtering means and part is reflected back from the filtering means, and in which method the filtering means is tuned to operate in a predetermined frequency band.
Base stations of radio systems often use filters implemented by cavity resonators, for example. The cavity resonators are placed in a separate resonator unit or a resonator case, for example. The purpose of the resonator case is to prevent the interfering signals from causing interference in signals to be filtered. The cavity resonators are tuned to the desired frequency upon commissioning of the radio system. The cavity resonators are tuned manually, for example. In tuning, an adjustment element located in the cavity resonator is adjusted with a suitable manual tool, for example.
Although the resonators are tuned to filter signals of a given frequency, they do not always stay in a predetermined frequency band, for which reason a desired filtering result is not obtained. The centre frequency of the resonators may change because of a change of temperature, for instance, and, consequently, the working range of the resonator changes.
Since manual tuning is time-consuming, a motor is commonly used for tuning the cavity resonators. The motor, such as a stepping motor, is connected to an adjustment element located in the cavity resonator to be tuned. The motor moves the adjustment element, whereby the filtering frequency of the resonator changes. Furthermore, the resonator unit may comprise measuring instruments which measure the cavity resonator before it is tuned. On the basis of the measurement data, a control signal is generated in the base station, and the control signal is used for controlling the motor to move the adjustment element of the cavity resonator.
The base station typically comprises several transceiver units, and each transceiver unit communicates with a specific cavity resonator unit. Each cavity resonator receives a signal from the transceiver unit and filters the signal it has received. The resonator unit comprises a measuring instrument which is located in the resonator unit or in the resonator case. The measuring instrument measures a signal which is reflected from the cavity resonator, and a control signal of the adjustment element of the cavity resonator is generated on the basis thereof. However, the prior art solution increases the size of the resonator unit, since implementing the solution requires a relatively large number of components. The number of components increases particularly when the measuring instruments are placed in the resonator unit. In addition, the practical implementation of the prior art tuning method is relatively complicated because of inefficient utilization of solutions that already exist in the base station.
It is therefore an object of the invention to provide a tuning method and a transceiver unit so as to solve the above problems. This is achieved by a tuning method of the type presented in the introduction, which is characterized by generating a test signal which is located in a predetermined frequency band and transmitted to the filtering means, generating signals of the same magnitude as the signal which advances to the filtering means and is reflected from the filtering means, and which signals are connected from the transmitting end to the receiving end of the transceiver unit, measuring the power of the signals that are connected to the receiving end, estimating the return loss of the filtering means on the basis of the power values measured, and when the return loss is greater than a limit value predetermined for it, the filtering means is tuned to carry out a more efficient filtering of the predetermined frequency band.
The invention also relates to a transceiver unit used in a base station which comprises, in addition to at least one signal-generating transceiver unit, a filter means which is separate from the transceiver unit and filters signals which are generated at the transmitting end of the transceiver, and of which signals part advances to the filtering means and part is reflected back, and which filter means is tuned to operate in a predetermined frequency band.
The transceiver unit is characterized in that the transceiver unit comprises a test signal generating means which generates a control signal and a test signal that is located in a predetermined frequency band, which test signal is transmitted further to the filtering means, a means which generates signals of the same magnitude as the signal which advances to the filtering means and is reflected from the filtering means, which signals are connected from the transmitting end to the receiving end of the transceiver, a measuring instrument which measures the power of the signals which are connected to the receiving end and estimates the return loss of the filtering means on the basis of the powers measured, an adjustment means which receives the control signal from the generating means and tunes the filtering means on the basis of the control signal it has received if a return loss value obtained as an estimation result is greater than the predetermined limit value of the return loss.
The preferred embodiments of the invention are disclosed in the attached dependent claims.
The invention is based on the idea that a signal which is reflected from a filtering means and advances to the filtering means is connected to the receiving end by means of a TRX loop used in a transceiver unit, whereupon it is possible to tune the filtering means by using the components which are located at the receiving end.
The tuning method and transceiver unit of the invention provide many advantages. The tuning method enables the use of base station components already implemented, whereupon the filtering means, such as a cavity resonator, can be tuned with fewer components. The structure of the base station can be simplified by using the existing components. The structure of the resonator unit, in particular, becomes more simple, because it is possible to remove some of the existing components therefrom. The implementation of a more simple structure is cost-effective as compared with the solution implemented by the prior art. Furthermore, because of fewer components required, the solution of the invention also reduces the probability of a failure in the base station. The method also enables the tuning of the filtering means during the operation of the base station.